Japanese Unexamined Patent Application Publication No. 2012-025257 (hereinafter referred to as Patent Document 1) has been proposed to obtain a mounting structure of a fuel tank that enables a fuel tank with a variable volume to be mounted on a vehicle body without obstructing deformation of the fuel tank ([0005] and Abstract). In order to realize this mounting structure, a fuel tank 14 in Patent Document 1 includes a tank upper-layer portion 16, a tank lower-layer portion 18, and a tank expansion and contraction portion 20. The tank expansion and contraction portion 20 expands in response to an increase in the internal pressure of the fuel tank 14, so that the volume of the fuel tank 14 is increased. A floor panel 34 includes a clearance portion 44 located above the tank upper-layer portion 16 so as to form an expansion space 46, and thus, an increase in the volume of the fuel tank 14 will not be obstructed (Abstract and FIG. 1).
As described above, in Patent Document 1, since the fuel tank 14 includes the tank expansion and contraction portion 20, the fuel tank 14 has flexibility, so that a flexible container is realized. In addition, in Patent Document 1, the expansion space 46 that allows the tank expansion and contraction portion 20 of the fuel tank 14 to expand is formed (Abstract and FIG. 1). In the case where the tank expansion and contraction portion 20 is made of a relatively soft material, it is assumed that the fuel tank 14 is guided to the clearance portion 44. In a configuration, such as that of a vehicle and the like, in which vibration and displacement are generated relatively frequently, there is a possibility that the tank expansion and contraction portion 20 and the like will become deformed, which in turn leads to deterioration of the fuel tank 14.
In addition, since the shape of the tank expansion and contraction portion 20 is inconsistent, in the case of trying to detect the remaining amount of fuel (or fluid) in the fuel tank 14 on the basis of the shape of the fuel tank 14, it is difficult to detect with high accuracy the amount of fuel (fluid).